I love Pylance type checking.
However, If I have a variable var: Union[None, T], where T implements foo, pylance will throw an error at:
var.foo() since type None doesn't implement foo.
Is there any way to resolve this? A way to tell Pylance "This variable is None sometimes but in this case I'm 100% sure it will be assigned
There are many ways of forcing a type-checker to accept this.
Use assert:
from typing import Union
def do_something(var: Union[T, None]):
assert var is not None
var.foo()
Raise some other exception:
from typing import Union
def do_something(var: Union[T, None]):
if var is None:
raise RuntimeError("NO")
var.foo()
Use an if statement:
from typing import Union
def do_something(var: Union[T, None]):
if var is not None:
var.foo()
Use typing.cast, a function that does nothing at runtime but forces a type-checker to accept that a variable is of a certain type:
from typing import Union, cast
def do_something(var: Union[T, None]):
var = cast(T, var)
var.foo()
Switch off the type-checker for that line:
from typing import Union
def do_something(var: Union[T, None]):
var.foo() # type: ignore
Note also that, while it makes no difference to how your type annotation is interpreted by a type-checker (the two are semantically identical), you can also write typing.Union[T, None] as typing.Optional[T], which is arguably slightly nicer syntax. In Python >=3.10 (or earlier if you have from __future__ import annotations at the top of your code), you can even write Union types with the | operator, i.e. T | None.